Development of dried uncharred leaves of Ficus benjamina as a novel adsorbent for cationic dyes: Kinetics, isotherm, and batch optimization

Adsorption is one of the efficient techniques for removing dyes from wastewater. The present work proposes the development of a biowaste-based low-cost, renewable, and porous adsorbent derived from the leaves of Ficus benjamina (FB). The dry leaves of FB are non-edible, and non-medicinal, producing a large amount of waste yearly. The adsorbent was employed for the removal of the cationic dyes rhodamine B (RhB), malachite Green (MG), crystal violet (CV), and methylene blue (MB). The adsorbent was characterized by FTIR, FESEM, EDAX, and BET analysis. The pore size, volume, and surface area were 2 nm, 0.035 cc g-1, and 24.2 m2 g-1, respectively. The batch adsorption studies were done by optimizing parameters like pH, the weight of FB, time of equilibration, and concentration of dye solution. Studies indicated that the maximum pH of adsorption for dyes depends on the pKa values of cationic dye. The optimum adsorption pH for RhB, and MG, were 2, 4, and 6 for CV and MB. The adsorption data were modeled using the non-linear Langmuir, Freundlich, Temkin, and Sips adsorption isotherm. The Freundlich and Sips isotherm showed better fitting of the data indicating the formation of a multilayer of the dyes on the surface. Adsorption kinetics was pseudo-second order highlighting the adsorption mechanism as a two-body interaction. Our studies indicated the maximum adsorption capacities of 9.92 mg g-1 for RhB, 11.01 mg g-1 for CV, 39.62 mg g-1 for MG, and 25.43 mg g-1 for MB. Thermodynamic studies at three different temperatures, 298, 308, and 318 K, show cationic dyes adsorption on adsorbent to be spontaneous and endothermic. Activation energies of RhB, MG, CV, and MB were found to be 90.14, 51.94, 58.74, and 24.22 kJ mol- 1, respectively, indicating chemisorption. Our studies indicated that the pulverized dry leaves of FB could serve as a good batch adsorbent for the cationic dyes in wastewater.